Electrophotographic marking is a well known method of copying or printing documents by exposing a substantially uniformly charged photoreceptor to an optical light image of an original document, discharging the photoreceptor to create an electrostatic latent image of the original document on the photoreceptor's surface, selectively adhering toner to the latent image, and transferring the resulting toner pattern from the photoreceptor, either directly to a marking substrate such as a sheet of paper, or indirectly after an intermediate transfer step. The transferred toner powder image is fused to the marking substrate using heat and/or pressure to make the image permanent. Finally, the surface of the photoreceptor is cleaned of residual developing material and recharged in preparation for the creation of the next image.
While many types of light exposure systems have been developed, a commonly used system is the raster output scanner (ROS) comprised of a laser beam source, a means for modulating the laser beam (which, as in the case of a laser diode, may be the action of turning the source itself on and off) so that the laser beam contains image information, a rotating polygon mirror having one or more reflective surfaces, pre-polygon optics for collimating the laser beam, post-polygon optics to focus the laser beam into a well-defined spot on the photoreceptor surface and to compensate for the mechanical error known as polygon wobble, and one or more path folding mirrors to reduce the overall physical size of the scanner housing. The laser source, modulator, and pre-polygon optics produce a collimated laser beam which is directed to strike the reflective polygon facets. As the polygon rotates, the reflected beam passes through the post-polygon optics and is redirected by any folding mirrors to produce a focused spot that sweeps along the surface of the charged photoreceptor in a straight scan line. Since the photoreceptor moves in a direction substantially perpendicular to the scan line, the swept spot covers the entire photoreceptor surface in a raster pattern. By suitably modulating the laser beam in accordance with the position of the exposing spot at any instant, a desired latent image can be produced on the photoreceptor.
To assist the understanding of the present invention, several things should be noted and described in further detail. First, the phenomenon known as scan line jitter is caused by the failure of pixels in successive scan lines to be precisely aligned with each other. It is common practice to position a photodetector element in the scan line path just ahead of the latent image area in order to establish an accurate measure of beam timing on successive scans. When the laser beam crosses the photodetector, a start-of-scan signal is produced which initializes the pixel clock controlling the data stream that modulates the laser beam. Second, in high quality imaging systems it is important that the laser beam have a stabilized intensity so that optimum exposure can be maintained. This enables optimization of the charging and development systems which are critical to producing high quality images. Having known beam intensities becomes even more important when multiple laser beams are used, such as in a color printer. Since the intensity of the laser beam from a laser source driven by a fixed current is strongly effected by operating temperature and changes with time due to aging, and since the output power of different laser sources driven by the same current can be quite different, the ability to dynamically regulate the intensity of the laser beams is important. Such regulation is typically implemented using a dedicated photodetector.
Normally, the production of the start-of-scan signal and the regulation of the laser beam intensity are carried out independently with separate photodetectors and separate preamplifiers, plus sufficient electrical support which includes connectors, wiring, and physical space for the two light sensing systems. The use of separate systems unnecessarily increases cost and both manufacturing and assembly overhead while potentially reducing system reliability. Therefore, a technique of achieving start-of-scan detection and dynamic beam intensity regulation using a single photodetector system for both functions would be beneficial.